1. Field of the Invention
The invention relates to the application of the techniques of nanoelectromechanical systems (NEMS) to ultrasensitive mass detection.
2. Description of the Prior Art
Micro-Electro-Mechanical Systems (MEMS) is the integration of mechanical elements, sensors, actuators, and electronics on a common silicon substrate through the utilization of microfabrication technology. While the electronics are fabricated using integrated circuit (IC) process sequences (e.g., CMOS, Bipolar, or BICMOS processes), the micromechanical components are fabricated using compatible xe2x80x9cmicromachiningxe2x80x9d processes that selectively etch away parts of the silicon wafer or add new structural layers to form the mechanical and electromechanical devices.
MEMS promises to revolutionize nearly every product category by bringing together silicon-based microelectronics with micromachining technology, thereby, making possible the realization of complete systems-on-a-chip. MEMS technology makes possible the integration of microelectronics with active perception and control functions, thereby, expanding the design and application space.
Whereas MEMS devices and processes are typically in the range of 1 to 100 microns, nanotechnologies contemplates processes a thousand times smaller, approaching a size just above or at the size of large molecules. Nanotechnologies thus contemplate processes and objects, which tend to be more chemical in nature than microelectronic. However, the availability of MEMS devices raises the question of whether these devices can be used in any advantageous way to measure and perform tasks at the next scale of size down, name in the 1 to 100""s of molecules or atoms. Such technologies are by analogy referred to as nanoelectromechanical systems (NEMS).
What is needed then is an apparatus and method by which such nano-processes can be accessed.
The invention is defined as a method for measuring microscopic magnitudes of mass and an apparatus realizing such a measurement, thereby enabling a novel approach to mass spectrometry. The method involves driving a nanoelectromechanical resonator at its resonance frequency, attaching the mass to be determined to the resonator by means of a chemical or physical adsorption process, and detecting changes in this resonance frequency due to the mass added to the vibrating element.
The apparatus further comprises a vacuum chamber for enclosing the nanoelectromechanical resonator and for directing the adsorbate molecules onto the resonator. The added mass is of the order of one or more macromolecules of matter adsorbed onto the vibrating element. Ideally, the added mass may be as small as one Dalton.
In the illustrated embodiment the vibrating element comprises a doubly clamped SiC beam. However, it is to be expressly understood that any nanoelectromechanical resonator now known or later devised is considered as an equivalent for the purposes of the invention. For example, torsional resonators, compound resonators with more than one vibrating element or arrays of resonators are other types of nanoelectromechanical devices which may be used.
In the illustrated embodiment vibrating element comprises a VHF microelectromechanical element. The higher the frequency, the better sensitivity is obtained.
The illustrated embodiment also comprises a plurality of baffles to shield the nanoelectromechanical resonator from radiation and includes means for thermally shielding or stabilizing the nanoelectromechanical resonator, such as a cryogenic bath surrounding the nanoelectromechanical resonator and thermally coupled thereto.
The apparatus can detect an added mass       δ    ⁢          xe2x80x83        ⁢          M      ~      C        ⁢                  M        tot            Q        ⁢          10              -                  (                      D            ⁢                          xe2x80x83                        ⁢                          R              /              20                                )                      ,
where xcex4M is the minimal magnitude of mass measurable by the nanoelectromechanical resonator, C is a constant determined by the geometry of the vibrating element, Mtot is the total mass of the vibrating element, Q is the resonant quality factor of the nanoelectromechanical resonator, and DR is the dynamic range of the nanoelectromechanical resonator and the measurement circuit. The added mass consists substantially of uncharged matter adsorbed to the vibrating element.
The invention is also defined as a method comprised of the steps of using the above defined apparatus to measure microscopic added masses to a nanoelectromechanical resonator.
While the apparatus and method has or will be described for the sake of grammatical fluidity with functional explanations, it is to be expressly understood that the claims, unless expressly formulated under 35 USC 112, are not to be construed as necessarily limited in any way by the construction of xe2x80x9cmeansxe2x80x9d or xe2x80x9cstepsxe2x80x9d limitations, but are to be accorded the full scope of the meaning and equivalents of the definition provided by the claims under the judicial doctrine of equivalents, and in the case where the claims are expressly formulated under 35 USC 112 are to be accorded full statutory equivalents under 35 USC 112. The invention can be better visualized by turning now to the following drawings wherein like elements are referenced by like numerals.